The present invention relates to an apparatus for inspecting the eccentricity of the core of an optical fiber preform and the ellipticity of the external shape of the optical fiber preform and a method for inspecting an optical fiber preform making use of the inspection apparatus.
An optical fiber comprises a core portion and a clad layer and permits the propagation of light rays along the core portion. For this reason, if two optical fibers are connected to one another, the cores of these two optical fibers should be precisely aligned. If the external shapes of two optical fibers coincide with one another, but either or both of the cores have eccentricity, the connection loss increases due to the discrepancy between the positions of these cores. For this reason, the preform as a starting material to be drawn into an optical fiber should comprise a core portion free of any eccentricity in the production of an optical fiber and the management thereof, it is very important to inspect an optical fiber preform as a starting material and to thus select an optical fiber preform comprising a core portion having low eccentricity from the foregoing standpoint.
Japanese Patent Application Publication No. 3-09060 discloses a method for determining eccentricity of an optical fiber which comprises observing the core of an optical fiber from two lateral directions perpendicular to one another to determine the coordinate of the off-centered core and correcting the coordinate for the magnification due to the curved side face of the optical fiber which serves as a lense and makes light rays refract to thus determine the degree of practical eccentricity of the core.
The inventors of the present invention practically determined the eccentricity of the core of an optical fiber preform according to the method disclosed in this patent. As a result, the degree of eccentricity of the core can precisely be determined if the optical fiber preform has an external shape which is an approximately true circle. However, if the external shape thereof is distorted, the eccentricity of the core cannot precisely be determined. Most of the distorted external shapes of optical fiber preforms are elliptic ones and this makes the determination of the eccentricity inaccurate. More specifically, if the core of an optical fiber preform having an elliptic external shape is observed from two lateral directions perpendicular to one another, the optical path length extending from the side face of the optical fiber preform to the core (optical path length corrected for the deviation due to the refractive index of the optical fiber preform) varies depending on the directions of observation, or the magnification of the curved surface of the preform which serves as a lense varies depending on the directions of observation. This accordingly leads to the inaccurate determination of the eccentricity of the core. According to the method disclosed in the foregoing patent, any accurate eccentricity cannot be obtained through the observation of an optical fiber preform from only two lateral directions.
Incidentally, if the core of an optical fiber preform is observed while immersing the preform in a matching oil as disclosed in Japanese Patent Application Publication Nos. 56-37493 and 57-21644, there can be ensured the elimination of any influence of the curved surface (i.e., side face) of the optical fiber preform as a lense (hereinafter referred to as "lense action of the curved surface") on the determination of the eccentricity of the core. This makes, needless, the requirement for any correction of the measured value for the magnification due to the lense action of the curved surface and, therefore, it is not necessary to take any change in curvature of the elliptic lateral surface of the preform into consideration. However, this method requires the use of a complicated apparatus since an optical fiber preform to be inspected must be immersed in a matching oil in this method and the method requires much labor to completely remove the matching oil adhered to the preform through washing after completion of the determination.